The present invention generally relates to tape automated bonded (TAB) circuits, and more particularly to a TAB circuit which comprise an encapsulation layer applied over lead/pad bonding regions of the circuit.
The rapid development of new and advanced microelectronic devices has created a corresponding need for improved circuit mounting structures. One type of circuit mounting structure currently in use is called a tape automated bonded circuit, commonly known as a "TAB" circuit.
TAB circuits were first researched and developed in the mid-1960's. As discussed in Rima, P. W., "The Basics of Tape Automated Bonding," Hybrid Circuit Technology, November 1984, pp. 15-21, a TAB circuit is constructed using a thin film carrier tape which is typically stored on large reels. The tape has a variable width of between 8 and 70 mm, and is approximately 5 mils thick. The length of each portion of tape used to form an individual circuit package is selectively variable, depending on the type of circuit to be made. The tape may be manufactured from a variety of different dielectric materials, including polyimide, and/or epoxy-glass compositions. Polyimide is generally preferred in that it has a high degree of mechanical strength, is capable of withstanding relatively high temperatures, and has a high coefficient of linear expansion similar to that of copper. It also has a relatively low coefficient of moisture absorption (about 3%).
There are numerous methods which may be used to construct TAB circuits. See, for example, U.S. Pat. No. 4,944,850 of John H. Dion et al., which is hereby specifically incorporated by reference for all that it discloses. One method (known as the "three layer process") involves the use of a thin, conductive foil typically manufactured of copper or copper alloy which is bonded to the tape using an adhesive known in the art. The foil is approximately 1.4 mils thick in a typical embodiment. In addition, an opening or window is physically formed through the center of each portion or "frame" of tape by chemical etching or other conventional means, including the use of a punch and die assembly. The foil is then etched to produce a conductive printed circuit pattern having beam-type inner leads which extend into the window.
In an alternative construction method commonly known as the "two layer process", a base layer of metal (e.g., copper) is directly sputtered or otherwise deposited onto the tape. Next, the tape substrate is run through an electroless bath of metal (e.g., copper) which deposits a very thin layer of metal onto the substrate surface and first metal layer. After window formation as described above, the top layer of metal is covered with a thin layer of photoresist which is imaged and developed, leaving a exposed metal pattern. The patterned substrate is passed through an electrolytic bath where a further metal layer is plated onto the exposed metal pattern. Resist materials are applied and subsequently etched to produce the completed product, as discussed in Dixon, T., "TAB Technology Tackles High Density Interconnections", Electronic Packaging and Production, pp. 34-39 (December, 1984).
As noted above, the metal used to create the circuit patterns normally involves copper or a copper alloy. These materials have a tendency to corrode which may adversely affect the operational capabilities of the circuit. To prevent surface corrosion, the circuit pattern is normally plated with a non-corrosive metal (e.g., gold, palladium, and/or rhodium.) Typically, the non-corrosive metal is electroplated onto the circuit pattern. Electroplating is a conventional process, which is normally accomplished by immersion of the circuit into a bath of metal solution, followed by the application of a current to the circuit. Simultaneously with the application of current to the circuit, a current of opposite charge is applied to the metal solution. As a result, metal from the solution is plated onto the circuit. So that electrical current may be applied to the circuit as described above, all of the circuit traces must be shorted together to ensure complete plating.
After plating, a selected electronic device, e.g. an integrated circuit chip (IC), is positioned within the window and secured therein by bonding the inner leads of the circuit to contact regions on the device.
Historically prior to inner lead bonding, the IC contact or "pad" regions, which are typically aluminum, would have gold bumps plated thereto. The bumping process involves multiple operations in addition to the standard wafer manufacturing process, increasing the risk of damage to the IC. The bumping process is also a relatively expensive process. In a more recently developed TAB manufacturing process, the gold plated TAB leads are bounded directly to the aluminum bonding pads of the IC, eliminating the need for pad bumping. One manner for achieving such direct lead/pad bonding is through one of a single point thermosonic bonding process which may be performed, for example, with a Hughes Model 2460-2 Thermosonic Single Point. Bender which is commercially available from Hughes Aircraft having a business address of 2051 Palomar Airport Road, Carlsbad, Calif., 92009.
When the pad bumping process is employed for achieving lead/pad bonding, the pad region of the IC is completely covered by the bump, protecting it from environmental degradation. The direct lead/pad bonding technique is generally preferable to bump bonding from a production cost standpoint. However, a problem with the direct lead/pad bonding technique is that it leaves the aluminum pad regions of the connected IC exposed. This shortcoming has been overcome by coating the side of the IC containing the pad regions with a dielectric encapsulation material which seals the aluminum bonding pads from environmental hazards.
Encapsulation material is generally chosen to match the coefficient of thermal expansion of the IC. An ideal encapsulation material provides protection from moisture and corrosive environments that the circuit may encounter. It also is low in alpha particle emissions, acts as a suitable alpha particle barrier, and has flow properties conducive to automated dispensing. Common encapsulating materials include silicone gels, thermoset epoxies, and polyimide-based coatings.
The encapsulating material is typically dispensed from a dispensing needle. The needle is positioned above the IC and is moved in an outwardly spiralling motion as the encapsulating material is dispensed. The encapsulating material is typically applied so as to cover the pad side of the IC. The encapsulation layer terminates near the edge of the pad side of the IC in spaced apart relationship with the periphery of the opening in the flexible substrate.
This encapsulation technique thus leaves the portion of each of the leads which is positioned over the opening in the flexible substrate exposed to the environment. However, since the leads, like the remainder of the traces, are coated with gold, this would not appear to be a problem.